Foundation, Concrete and Earthquake Engineering

Quality of water in concrete Mix

Introduction

Concrete is a chemically combined mass which is manufactured from binding materials and inert materials with water. It is most popular construction material due to its unique durability and reasonable strength; more interestingly can be modified and designed for wide range of strength requirements and set under variable environmental conditions. Cement is the most important material of concrete which is produced at the cost of environmental emission of CO2; to produce 1 tonne cement nearly 900 kg CO2 is released in the environment. So such an energy intensive materials constitutes concrete which may be seriously affected by (both strength and durability point of view) by relatively available and cheap but essential element water; more precisely impurities in water. Quality of mixing water are mainly considered for performance of concrete in both fresh and harden state.

Impurities in mixing water intervene the setting time of the paste and may produce detrimental effect on strength and durability of concrete also. When impurities are chemically active, they may take part in the chemical reaction contributing significant change in setting, hardening and development of strength of concrete. More over health hazard during handling these water should carefully considered. In this regard past performance of a particular source of water can be used to evaluate suitability of water; if not available, some testing inevitable to evaluate water for setting time, compressive strength and durability.


Function of Water in Concrete

Three water serves the following purpose:
  1. To wet the surface of aggregates to develop adhesion because the cement pastes adheres quickly and satisfactory to the wet surface of the aggregates than to a dry surface.
  2. To prepare a plastic mixture of the various ingredients and to impart workability to concrete to facilitate placing in the desired position and
  3. Water is also needed for the hydration of the cementing materials to set and harden during the period of curing.
The quantity of water in the mix plays a vital role on the strength of the concrete. Some water which have adverse effect on hardened concrete. Sometimes may not be harmless or even beneficial during mixing. So clear distinction should be made between the effect on hardened concrete and the quality of mixing water.

Potable water as mixing water


The common specifications regarding quality of mixing water is water should be fit for drinking. Such water should have inorganic solid less than 1000 ppm. This content lead to a solid quantity 0.05% of mass of cement when w/c ratio is provided 0.5 resulting small effect on strength.

But some water which are not potable may be used in making concrete with any significant effect. Dark color or bad smell water may be used if they do not posses deleterious substances. PH of water to even 9 is allowed if it not tastes brackish. In coastal areas where local water is saline and have no alternate sources, the chloride concentration up to 1000 ppm is even allowed for drinking. But this excessive amount of alkali carbonates and bicarbonates, in some natural mineral water, may cause alkali-silica reaction.

Acceptable source of water


Besides potable water, various new and existing sources are available for mixing water which can be used for complete and partial replacement of valuable potable water. This includes

• Ground water

• Reclaimed water

• Treated water from municipal sewer

• Waste water of ready-mix concrete plant etc.

In many regions of the world there have scarcity of water like Dubai and Qatar and the local authorities are looking for new sources and reused water. There treated water are used for agricultural requirements and daily needs for construction industry. like washing aggregates, as concrete mixing water and curing of the same. Water from river and sometimes even sea are considered suitable if it is free from brackish matter. In arid regions, brackish groundwater is mixed with desalinated water and considered suitable for concrete production and for concrete slurry too.

Sampling guideline for mixing water


In addition to testing on constituent of concrete like aggregate cementitious materials and admixtures etc., testing of water is an important part of quality control of concrete. A systematic testing schedule for water testing yields higher efficiency of concrete and assure good performance in regard of strength and durability.

Important thing to remember is that water can be changed by chemical, physical or biological reactions; such modification may occur during sampling and at the time of analyzing. So it should be tested before using in concrete.

Collection of sample


The location of sampling should be at mid-stream and extracted from mid depth, as far as possible. When there have obstructions or major discharges are fall into river, the sample should be taken in downstream of discharges by a distance more than 100m in case of small stream; in a word, the site should be such that no change in water in the stream are seen with naked eyes. In case of wide river at least three samples should be taken along the cross section.

Sampling of waste water


When water from narrow effluent channels of treated sewers are to be tested, the sample should be taken from one third depth of water neither skimming the top surface nor scrapping the bottom. It is important to locate site having sufficient flow so that no nearby deposition is occurred. Caution should be taken during sampling to keep in-situ condition of dissolved gas i.e. must not be agitated to liberate dissolved gasses or to cause some degree if aeration.

It was observed that sewage flows are often intermittent which requires to collect samples an hour interval throughout 24 hours. At room temperature waste water generally decompose rapidly, so test set-up for certain parameter should be available at site. These are as follows:

  • Dissolved oxygen
  • Residual chlorine
  • Sulfides
  • PH
  • Nitrites etc.
For some tests addition of preservatives just after collection of water will be enough.


Can ready-mix concrete washout water be used in water?

It is recommended in AS 1379 and ASTM C94, that water used in washout operation in ready-mix concrete plant can be used in concrete as mixing water. 
Only requirement is to store it in such a way that contamination from deleterious matters is prevented and water is collected from storage outlet. Water should conform ASTM C 1602; the sources and testing frequencies and other requirements of testing to qualify water sources. According to ASTM C94 water may be water itself or may be ice or any forms of moisture on the aggregate surface and wash water remains in the drum of truck mixer can be used for concreting next batch.

Determination of Suitability of Mixing Water


A simple way of determining the suitability of such water is to compare the setting time of cement and the strength of mortar cubes using the water in question with the corresponding results obtained using known suitable or distilled water. About 10% tolerance is generally allowed. Such tests are recommended when water for which no service record is available containing dissolved solids in excess of 2000 ppm or, in excess of 1000 ppm. When unusual solids are present a test is also advisable.


Quality Parameters
Maimum Limit (ppm)
Chlorides
500
SO3
1000
Alkali Carbonates
and Bicarbonates
1000
Turbidity
2000

The effect on concreting for different types of contamination or impurities are described below:

Suspended Solids

Mixing water which high content of suspended solids should be allowed to stand in a setting basing before use as it is undesirable to introduce large quantities of clay and slit into the concrete.

Acidity and Alkalinity

Natural water that are slightly acidic are harmless, but presence of humic or other organic acids may result adverse affect over the hardening of concrete. Water which are highly alkaline should also be tested.

Algae

The presence of algae in mixing water causes air entrainments with a consequent loss of strength. The green or brown slime forming algae should be regarded with suspicion and such water should be tested carefully.

Sea Water

Sea water contains a total salinity of about 3.5%(78% of the dissolved solids being NaCl and 15% MgCl2 and MgSO4), which produces a slightly higher early strength but a lower long-term strength. The loss of strength is usually limited to 15% and can therefore be tolerated. Sea water reduces the initial setting time of cement but do not effect final setting time.

Chloride

Water containing large amount of chlorides tends to cause persistent dampness and surface efflorescence. The presence of chlorides in concrete containing embedded steel can lead to its corrosion.

Moisture Content of Aggregate


Aggregate usually contains some surface moisture. Coarse aggregate rearly contains more than 1% of surface moisture but fine aggregate can contain in excess of 10%. This water can represent a substantial proportion of the total mixing water indicating a significant importance in the quality of the water that contributes surface moisture in aggregate.

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